import tensorflow as tf

x, y = tf.Variable(1, 'x'), tf.Variable(1, 'y')
cond = tf.placeholder_with_default(False, ())

#  first
x_inc = tf.assign_add(x, 1)
y_inc = tf.assign_add(y, 1)
op = tf.case({cond: lambda: x_inc},
             lambda: y_inc)

#  second
# op = tf.case({cond: lambda: tf.assign_add(x, 1)},
#              lambda: tf.assign_add(y, 1))

sess = tf.InteractiveSession()
sess.run(tf.global_variables_initializer())
sess.run(op, feed_dict={cond: False})
sess.run(op, feed_dict={cond: True})
print(sess.run([x, y]))  # => first: [3, 3], second: [2, 2]